Adds a test that simulates a scenario where the first buffers after
a segment can't be decoded and the decoder asks for those frames
to be released. The videodecoder base class should make sure that
the events attached to those first buffers are pushed even if the
buffers aren't going to be.
https://bugzilla.gnome.org/show_bug.cgi?id=721835
Add a simple playback test that makes sure that video decoder pushes
buffers in the same order it receives and that it respects the
set timestamps and durations
